The present invention relates to a light source device that includes a light emitting diode (LED) as a light source. The present invention specifically relates to the light source device that includes a LED chip with a fluorescent substance (e.g., a phosphor).
In case that an illumination device for a specific area is needed, an LED device, which converges a light from the LED with a lens, is used as a spot light. Meanwhile, an LED device, which emits white illumination, is recently developed and put to practical use.
A typical white LED includes a blue LED chip and a fluorescent substance that converts a part of a blue light from the blue LED chip and emits yellow luminescence. The blue light, which is emitted from the blue LED chip, and the yellow luminescence, which is converted and emitted by the fluorescent substance, are mixed and observed as white illumination. Because there is a difference between a wavelength of the blue light and the yellow fluorescence light, a color split is observed around a periphery of the white illumination. The fluorescent material works as a light diffusing agent. Therefore, if a sufficient amount of the fluorescent material is used in a white LED, a light emitted from the white LED is well-equalized and a color split around a periphery of the white illumination is reduced. However, an amount of the fluorescent material should be decided by considering a balance between a blue light and a yellow luminescence. Because the amount of the fluorescent substance is not enough for diffusing when the white LED emits white illumination, a good light diffusion cannot be expected. When the amount of the fluorescent substance is small in the white LED, a light diffusion rate is reduced. Then, an illumination from an LED device is bluish white at the center of the illumination and yellowish white at the periphery of the illumination. Furthermore, when the light from the LED device is converged by a lens, a color split will occur because of an aberration of the lens.
A combined lens is generally used in order to improve such a color split. The combined lens can improve the color split around the center of an illumination, but cannot improve it around the periphery of the illumination. On the other hand, a lens can include a diffusing agent for diffusing a light from the LED device. In this case, however, the lens cannot converge the light from the LED well.